A conventional SOFC system has been proposed, in which: an oxide ion conductive solid electrolyte is used as an electrolyte; and electrodes are provided at both respective sides of the electrolyte. This SOFC system is configured to generate electric power at high temperature with high efficiency in such a manner that a reformed gas obtained by reforming a raw material such as city gas (13A) is supplied to one of the electrodes, and air (oxygen) is supplied to the other electrode. By a chemical reaction between oxide ions having passed through the oxide ion conductive solid electrolyte and hydrogen ions, this conventional SOFC system generates steam and carbon dioxide to generate electricity and heat. The generated electricity is taken out to an outside of the SOFC system to be supplied to various electric power loads. Further, the heat generated in the electric power generation is used to heat the raw material, the air for the electric power generation, water for the reforming, and the like (see PTL 1, for example).
In a fuel cell module included in the SOFC system according to PTL 1, the reformed gas generated in a reformer is supplied to a fuel cell unit through a fuel gas supply pipe. Further, air is preheated by heat exchange with an exhaust gas using an air heat exchanger to be introduced to an electric power generating chamber through an air introduction pipe. The fuel cell unit is constituted by a plurality of cells each including: a cylindrical inner electrode layer (fuel electrode) in which a fuel gas channel is formed; a cylindrical outer electrode layer (air electrode); and an electrolyte layer provided between the inner electrode layer and the outer electrode layer. In the SOFC system according to PTL 1, the reformed gas unconsumed in the electric power generation and the air unconsumed in the electric power generation are combusted in a combustion chamber, and heat of this combustion heats the reformer and the air heat exchanger configured to preheat the air. Further, the SOFC system according to PTL 1 is configured to prevent an unburned gas or an incomplete combustion gas such as carbon monoxide, generated by the combustion in the combustion chamber, from being discharged to the outside. To be specific, the SOFC system according to PTL 1 is configured such that while giving priority to such a control operation that the temperature of the fuel cell stack falls within an appropriate temperature range, the discharge of the unburned gas or the incomplete combustion gas is suppressed by correcting the supply amount of air for the electric power generation.
Further, a method of starting up a fuel cell apparatus has been proposed, by which whether or not a gas utilized in combustion is ignited can be determined for the purpose of suppressing the generation of the unburned gas and the like (see PTL 2, for example). According to the fuel cell apparatus of PTL 2, when the gas is ignited by an ignition heater, and the temperature of a combustion region increases from a predetermined temperature T0° C. to not less than a temperature T1° C. within a predetermined period of time, the fuel cell apparatus of PTL 2 determines that the gas has been ignited. When the ignition is not determined within a predetermined period of time, the fuel cell apparatus of PTL 2 determines that the gas has not been ignited. As above, the fuel cell apparatus according to PTL 2 can determine whether or not the gas is ignited by the ignition heater, to suppress the discharge of the unburned gas.